"I have not seen my father for a year."
Translation:Yo no he visto a mi padre por un año.
Because padre comes after the verb and is a regular direct object. If you had said the sentence in reverse: i.e. A mi padre, no lo he visto... This sentence would need to be in context, and used for emphasis, eg: I saw my mother yesterday, but my father, I haven't seen him for a year.